1. Field of the Invention
The present invention relates to a process for the preparation of foamed polyolefin beads which have a uniform cell size over their entire cross-section.
2. Description of the Background
A number of processes are known for the preparation of foamed polyolefin beads by means of dispersion foaming. For example, in DE-A-21 55 775, a dispersion of polymer particles in a liquid dispersing agent is heat treated under pressure, and foamed with subsequent release of the pressure. Foaming is explained by the fact that the dispersing agent infiltrates hollow cavities in the polymer and acts as a blowing agent when ejected into a low pressure space. The polymer should contain 10 to 70% by weight of a filler in order to facilitate the infiltration. Prefoamed beads having a fine and uniform cell structure are obtained by this method. As the examples show, however, the expansion rate is non-uniform. In practice, it has been found that processing of these prefoamed beads to foamed shaped articles is difficult because of the high filler content and the low cell diameter.
Foaming specifically of ethylene/propylene random copolymers is described in EP-A-0 053 333. A system is used there which comprises polymer particles, water as the dispersing agent, a solid dispersion auxiliary and a volatile blowing agent.
It is known from EP-A-0 071 981 that foamed polypropylene beads can be welded to form dimensionally stable moldings if the size of the gas-filled cells within the foam is adjusted such that the cut surface contains no mere than 300 cells per mm.sup.2, the molding having a density of 0.026 to 0.060 g/cm.sup.3 and a latent heat of crystallization of 9 to 28 cal/g. However, this specification gives no instructions as to what technical measures are necessary to obtain these cell characteristics.
It is furthermore known that, in order to maintain a constant expansion rate of the foam, the let-down pressure must be kept constant such that the gas volume increasing in the reactor as a result of the dispersion being let down is compensated by topping up with inert gases. Letting down in EP-A-0 075 897 is thus carried out under a pressure which is at least 0.7 times the partial pressure of the blowing agent, and preferably greater than or equal to the partial pressure of the blowing agent. The doctrine of BP-A-0 095 109 furthermore is that, in order to achieve a uniform cell structure and a constant expansion rate, not only must the temperature be kept constant as accurately as possible, but the internal pressure and the partial pressure of the blowing agent must also be kept constant.
Nevertheless, by topping up with cold inert gases or cold blowing agent, which must evaporate by absorption of heat in the reactor, depending on its nature, the contents of the reactor cool in an undesirable manner. According to EP-A-0 290 943, this cooling is avoided by the level of liquid in the reactor and therefore the pressure and temperature remaining constant by means of a supply of heating fluid. Since considerable amounts of heating fluid must be supplied, however, this process is associated with a considerable expenditure of energy.
According to the prior art, considerable efforts must therefore be made to achieve a uniform cell structure of the foamed beads. A need therefore continues to exist for an improved method of achieving a uniform cell structure of foamed beads.